Build Win32 with Yasm rather than MASM.

Although x86masm.pl exists, upstream's documentation suggest only x86nasm.pl is
supported. Yasm seems to handle it fine with a small change.

Change-Id: Ia77be57c6b743527225924b2b398f2f07a084a7f
Reviewed-on: https://boringssl-review.googlesource.com/2092
Reviewed-by: Adam Langley <agl@google.com>

crypto/CMakeLists.txt

@@ -12,12 +12,16 @@ else()
 	if (CMAKE_CL_64)
 		message("Using masm")
 		set(PERLASM_STYLE masm)
+		enable_language(ASM_MASM)
 	else()
-		message("Using win32")
-		set(PERLASM_STYLE win32)
+		message("Using win32n")
+		set(PERLASM_STYLE win32n)
+
+		# On 32-bit, upstream supports only NASM, not MASM. We'll use Yasm, specifically.
+		set(CMAKE_ASM_NASM_COMPILER "yasm")
+		enable_language(ASM_NASM)
 	endif()
 	set(ASM_EXT asm)
-	enable_language(ASM_MASM)
 endif()
 
 function(perlasm dest src)

crypto/perlasm/x86nasm.pl

@@ -83,7 +83,15 @@ sub ::file
 %ifidn __OUTPUT_FORMAT__,obj
 section	code	use32 class=code align=64
 %elifidn __OUTPUT_FORMAT__,win32
+%ifdef __YASM_VERSION_ID__
+%if __YASM_VERSION_ID__ < 01010000h
+%error yasm version 1.1.0 or later needed.
+%endif
+; Yasm automatically includes @feat.00 and complains about redefining it.
+; https://www.tortall.net/projects/yasm/manual/html/objfmt-win32-safeseh.html
+%else
 \$\@feat.00 equ 1
+%endif
 section	.text	code align=64
 %else
 section	.text	code